The vagus nerve is one of the principle conduits of sensory information from the airways to the brain, and its integrity is required for normal respiration. Vagal neurons detect stretch in the lung, inflammatory mediators, and irritants, and drive reflex outputs including cough, bronchoconstriction, and airway secretion. Vagal sensory neurons also contribute to pathophysiology in animal models of asthma, but the mechanisms by which vagal sensory neurons detect and encode stimuli and their contributions to asthma pathogenesis are poorly understood. As a result, current therapy remains unable to specifically target the neurogenic component of airway disease. In preliminary data, we used molecular and genetic techniques to identify non-overlapping subsets of vagal sensory neurons that target the lung. These neuronal subsets are defined by expression of either P2RY1, a purinergic G protein-coupled receptor (GPCR), or NPY2R, a neuropeptide Y GPCR. We adapted tools to map, image, and activate these neuron populations and found that they exhibit strong and opposing effects on breathing (Cell, 2015). When activated, P2RY1 neurons induce apnea, while activation of NPY2R neurons causes rapid, shallow breathing. We further found that P2RY1 neurons are capsaicin-insensitive, express the mechanosensitive ion channel Piezo2, and are fast-conducting A-fibers. In contrast, NPY2R neurons are largely capsaicin-sensitive C fibers. Based on these results, I hypothesize that NPY2R neurons are dedicated irritant receptors, while P2RY1 neurons serve a regulatory role. However, the stimuli to which these neurons respond has never been directly tested. Furthermore, their contribution to airway defense reflexes and disease have not been tested, despite strong parallels between NPY2R neurons and previously described vagal pulmonary nociceptors. In this application, I propose a series of experiments to define the contribution of NPY2R neurons to a mouse model of allergic asthma. Next, I introduce a novel in vivo calcium imaging preparation that has been developed and validated in the Sponsor's laboratory, which I will use to query the response properties of vagal P2RY1 and NPY2R neurons. These experiments will define a vagal sensory circuit from sensory detection in the lung to reflex motor output and establish the role of this circuit in an important model of airway disease. Understanding what these sensory neurons detect and control will advance our understanding of airway disease and may provide a basis for novel cellular and molecular therapeutic targets.